One of the premiere mysteries of modem science has related to the question of why invertebrates possess the ability to undergo CNS regeneration whereas the CNS of most vertebrates cannot. In a quest to provide some insight into this mystery, many investigators have sought to delineate some of the key players that promote neurite outgrowth during both development and regeneration. One of the first of such players to be identified was a protein called GAP-43, a neuron-specific, acidic, phosphoprotein associated with neurite outgrowth that occurs during development, regeneration, and synaptic plasticity. Importantly, GAP-43 expression is not upregulated when the CNS of vertebrates is injured, suggesting that this protein plays an important role in the regenerative process. In addition to GAP-43, several other proteins have been identified that play a role, not in axonal outgrowth, but in guidance of the growing axon to its final destination. One such family of proteins, the semaphorins, functions as chemorepulsive and/or chemoattractant molecules that are necessary for correct neuronal pathfinding. Significantly, some semaphorin family members continue to be expressed at low levels in the post-mitotic adult vertebrate brain. It is intriguing to postulate that continued expression of semaphorin family members in the mature vertebrate may lead to suppression of GAP-43, and when injury occurs, GAP-43 cannot be turned on again to promote axonal outgrowth. We propose to begin to dissect this problem using a molecular biology approach to understand the possible connection between GAP-43 and semaphorin. It is expected that completion of the specific aims set forth in the attached proposal will shed light on this very important issue and contribute significantly to advance our current knowledge about processes that impact nervous system development and regeneration, as well as tissue repair.